Device for reproducing magnetic records



Jan. 2, 1951 1.. L. BURNS, JR

DEVICE FOR REPRODUCING MAGNETIC RECORDS Filed Dec. 30, 1948 INVENTOR I 51/5! 501?: JR.

ATTORNEY Patented Jan. 2, 1951 DEVICE FORREPRODUCING MAGNETIC RECORDS Leslie L. Burns, Jr.,-'Princeton, N. J assignor to Radio Corporation of America, a corporation of Delaware Application December 30, 1948, Serial No. 68,227

' 6 Claims. (07.. 179-1002) My present invention relates to improvements n the 'art of transcribing magnetically recorded signals. Y

' The frequency response-characteristic of pres- 'ent day magnetic phonographs (or telegraphones)' as such instruments are sometimes called) is not linear, but is a function of the frequency of the recorded signals. The low fre quency signals are attenuated and it is therefore necessary to provide a disproportionate gain for the low audio frequencies in order to restore them to their originalvolume. This'gives rise to60 cycle hum problems and to microphonic feed-back problems and hence complicates the electrical and mechanical design of the instrument.

Accordingly, the principal object of my present invention is to provide a magnetic phonograph having a substantially linear frequency response characteristic.

Another and related object of my invention is to provide a novel and sensitive method of, and apparatus for, picking-up and translating magnetically-recorded signals, and one applicable to telegraphone records of standard form (tape, wire, disc or cylinder) and recorded in any conventional manner (i. e. longitudinal, transverse or perpendicular magnetization).

The foregoing objects are achieved in accordance with the method of my invention by gencrating electrical oscillations and varying the frequency of the said oscillations in accordance with the magnetic intensity of the signals at successive points on the sound track upon which the signals are recorded, and then demodulating said frequency-varied oscillations and converting the said demodulated electrical signals into audible signals.

My invention is described in greater detail in connection with the accompanying partly diagrammatic view of a magnetic phonograph embodying the invention.

In the embodiment of my invention which I have selected for illustration, I designates, generally, a reelable tape-like record, having a powdered iron sound-track 3 thereon, which will be understood to be wound to run (between a storage reel and take-up reel, not shown) in a path which spans the non-magnetic gap 5 between the polar extremities I and 9 of a magnetic transducer II of a construction dictated by the principle of my invention. In the instant case the transducer It comprises a split ring or rectagular yoke comprising two preferably laminated h-shape magnetic structures Ila, IIb, arranged in juxtaposition and provided with a cross-arm i3 constituted of a material which exhibits a relatively large (as compared to powdered iron) permeability change when the degree of saturation of the material is changed. Such material may be a homogeneous crystalline material composed of ferric-oxide and the oxide of another metal, this material being known in the art, and being referred to hereinafter, as ferrite.

' The signals on the sound track 3 pass to one pole piece or polar extremity I through the high permeability ferrite cross-arm or core I3 to the other pole piece 9 and back to the sound track. The variations in magnetization of the sound track, which represent the recorded signals, are thus impressed on the ferrite core I3. The variations in the intensity of the magnetic flux, from the sound track, passing through the ferrite core I3 change the permeability of the ferrite in accordance with the previously recorded signals. As shown in the drawing; there is a radio frequency coil I5 wound on the ferrite arm or core 13; This coil comprises the frequency determining'inductor of a vacuum tube oscillator II operating at any conveniently low radio frequency. As the inductance of this coil I5 changes, as a result of the changes in permeability of the fer rite material, the frequency of the oscillator changes. These changes in frequency are then converted to audio signals in any convenient manner, e. g. by means of a balanced discriminator circuit I9 of the type disclosed and claimed in U. S. Patent 2,121,103 issued to S. W. Seeley on June 21, 1938. Alternatively, the audio signals may be taken off across a resistor 2| connected between the grid 23 and cathode 25 of the vacum tube oscillator ll. In either event the audio signals are preferably applied to an audio amplifier 21 prior to being applied to a loudspeaker 29 or other suitable signal reproducing apparatus.

It is of course apparent that the magnetic signals upon the sound-track 3 would be erased if the R. F. oscillations, applied to the ferrite core I3 through the coil I5, were permitted to impinge upon the sound track. This possibility is minimized by constructing the L-shape parts IIa -I lb of the transducer of soft iron and by making the ferrite core I3 of open work construction so that the latter comprises a complete magnetic circuit for the radio frequency flux.

I apply a magnetic bias to the ferrite core I3 so that the transducer may be operated on a linear and sensitive portion of the permeability vs. magnetic flux curve. In the instant case this bias is provided by a permanent magnet 3| which is conveniently mounted within the central aperture 33 of the core l3. In order to prevent the flux of this magnet 3| from reaching the sound track 3 and erasing or distorting the signals thereon I provide a balancing coil 35 around one arm of' the yoke. This coil 35isisupplied with direct currentfrom amsource. exemplifiediby a battery 31 and potentiometer 39. Recording may be accomplished by applying audio signals from a source, exemplified by a microphone 4i and amplifier 43, to the balancing co'i13'5 throughw. switch 45 thus making it unnecessary to provide a separate magnetic transducer-for.thisipllrnose.

I claim as my invention:

1. In a magnetic phonograph, magnetic means of variable permeability defining :a flux path having a non-magnetic gap therein, said magnetic means including a pair of pole pieces-and a ferrite core member l intermediate said pole piecearemotezfrom:sairhgappbiasingmeans assm .ciatedwith :said ferrite member means* for moving a :magnetically I recorded sound-:track 1 across said-gap, :and an oscillation-generator rhavingaa frequency adetermining inductor -.coupled to said ferrite meni-ber'and subject to theeffectsof variations 5 in theapermeability in"sa-id flux patlr incident :to the movement: ofzsaid magnetically recorded sound track=aeross-said gap.

42. :A \ma-gnet-ic gphonqgraph 1 comprising, :mag-

netic 4 meansa defining a fiux p ath .havingza: non- -:xnagnetic gapa-thereintsaidimagneticLmeans-comprising aipairof pole;pieceswandeamore member intermediate: said pole lpieces .remote from said igap, :said more member i'being :constituted 10f ea material exhibiting a relatively:large permeability change:fen-:arciatively smallzchange .in.=.flnx, means fonmoving-amagnetieally. recorded soundtrack across said :gap, .'ian1.oseillation ggenerator generator.

3. A sound-head for picking-up signals from a magnetic record, said sound-head comprising a bi-part magnetic structure having polar extremities defining the boundaries of a non-magnetic gap across which said record may be moved, a core portion constituted of a ferrite material mounted remote from said gap and vconnecting 'ithejgpartsrof saidIbi-partzma-gnetic structure, and coil meansmounted upon said core portion for :impressing radio frequency oscillation upon said ferrite material.

:4. "The invention as set forth in claim 3 wherein said core serves as a support for a permanent rma-gnet.

5."lTii'erinvention as set forth in claim 4 and wherein said-magnetic structure serves as a sup- .port,for-=a..coi1 thr0llgh which direct current maybe applie'dto said' structure.

6..11'11'2. magnetic phonograph, magnetic means of variable permeability defining a flux path havingea inonemagnetic g-ap therein, said ma netic meansecomprisingra; pair 1 Qf -SGf-t; iron pole pieces and a ferrite core member intermediate said pole 1 pieces? remcteifromssaidlgapasaid ferritermember REEERENCES CITED "The ifoilovving rref'erences sane cof :record :in (the file of this patent:

FOREIGN PATENTS Country Date ,Norway 1 May1;20,.=1946 Number 

